Signal amplifying system and method



July 28, 1931.

H. s, BAIRD SIGNALYAMPLIFYING SYSTEM AND METHOD Filed April 30 1929 17122811707". I ii} Patented July 28, 1931 UNITED TATES .HOLLIS s. BAIBD, or BOSTON, nesseonusnrrsg Assmnon VISION scares-Arrow, 0F Bosrom MASSACHUSETTS, A, eonronnrrou or 1mm wnan sIoimnAMrLmYme 'SYSTEM,ANDMETHOD a mputati n filed a 'miao, 1929., 'Serial'No. award 1 T This invention relates to signal amplify ingsystems especially adapted to amplify an input signal to a high-degree over a Wide range of frequencies with a minimum ofdistorti on. The amplifying system is, here illustrated in connection with the amplificationof televisionsigna'ls although it is'not necessari-= 1y limited to such use.

For the amplificatbn which will arrrpl ify the input signal throughout a wide range of frequencies Without appreciable distortion; otherwise a distorted image will result The amplifying system, a especial'l'y at the transmitter, must also have u a very high overall amplification-due to the" veryjweak voltage change which is provided by the -Visua'l-Wa-Veresponsive device, usually a photo-electric cell. An amplifying systemf capable of fulfilling the above requirements constitutes one of the 'obj ects of the present invention;

The usual amplifying system embodiesthe usual multi-element thermionic tubes c01 1- Ine'eted in cascade, wherein the output or plate circuit of one tu-be influences the input "or gridcircuit of the nextsucceeding tube. A resistance amplifying *systein involving the use of a p'luralityof thermionic tubes, Where in resistors arein'cluded in the -plate or-out- I put circuits of the-tubes,- the variation of volta'ge 'a'crcss'the resistors varyingthe potential of the grids F of the next succeeding I tubes is Well 'kHOW-ll and'has the property of gampli fying the input signal with as little distortion as an amplifying systempreviously know-n. I owever,with' this type ofempfli fier, some means must be provided to mainta-in the' grids o fthe tubes ata suitable negative potential or bias and ,ito keep the high potential'of theplates isolated from the grids; A 7 common method involves the use of a" 0011 pl ing or grid condenser and grid leak which are interposed between the grid and the plate circuit of the last preceding tu'bes. Such grid condensers and grid leaks are 'not satis- 1 factory especially for the amplification of televisipn signals because of-the distortion of the signziliat various frequencies an'd the hapmzard. Voitege fluctuations this result-in ofjtelevision signals especially, an amplifying system is desirable I y l fro, srzonrwnvu e 1 their use, especially with amplifications, i I and great tendency to overload with increas ing amplification. In ahigh gain amplifier- When-not usiugjthe eoupling" condenser one-of thebiggest sources of distortion i's'elimrna'ted, l

the leakage in'this-oondenser arising in each stage and, in the initial stage, may as great as the signal itself. I

One ofthe o-b'e'cts of the present inven-tionj' is the fPFOV-IlSlOllO-f an improved amplifying j system of this general eharaeter wherein the p grid of one tube is made responsive to the variation' of potential across are-sister in the plate cireuit of the last preceding-tube and 'Wherein theg-rid'is maintained 'at a proper biasing potential from the plate power by means of the normal plat'eeurre'nt.

Afurtherobj ect of theapresent'invention is theprovision of a practical meansof 'tai ning grid bia's in a direct coupled amplifien, so. that the amplifier may use stam'lard" In carrying out these objects of theinvenblocks of plate battery-end operate eover Y periods of time with a minimum of adj ustj ment.

*tion, I provide 'aseparateeouree of energy," or plate battery, for the plate'oircuit 'of each I amplifying stage of the system, which plate battery is divid'edintotwo sections connected nth-rough I a resistor which is variable; at least each plate eircuithas its, full high potential :and yet due to the drop across the resistor caused by the normal plate current flowing therethrough, the '-grid oan be adjustedte have its normal'bias and yet-respond'to varia I tions of; voltage across the resistor.

A ,further object of the invention as n i improve signal amplificationswmbs.

toms and methods.

The -fig-ure is a-circuit diagram of a: signal amplification system embodying the piesent invention especially adapted ior ampl fym v the; signals from a photseletnerei it mph-flying system; here shows som 5 or filaments 16. The thermionic tubes of the first four stages are of the shielded grid type having the shields or screen-s 18 surrounding the plates 14; and interposed between the plates and grids. The apparatus of each stage can be enclosed in a metal casing 20 to prevent electro-magnetic coupling between the stages. The filaments of the stages are heated from a suitable source of power here shown as the battery 22. The signal to be amplified is caused to affect the potential of the grid 12 of the first stage tube. As here shown, the input circuit of the first stage is associated with one or more photo-electric cells 24 provided with a suitable source of potential 26. The grid is maintained at a suitable bias by means of the battery 28 and the variable potentiometer 30 which is connected to the grid through the grid leak 32. The tube 10 of the last stage 6 is or can be a three element tube of the power amplifier type and has its output circuit 33 connected with the load 34.

In accordance with this invention, the anodes of the stages a, b, 0 and d are separate- 1y supplied with sources of direct current power, each source of which consists of a battery having two separate sections 38 and 380; respectively, arranged in series in the output circuits of the tubes and connected by an interposed resistor 40 which is variable at least in part. The grid of each tube of an amplify ing stage is connected by a conductor 42 to the output circuit of the preceding stage at the junction between the power source or bat- 40 tery 38a and the resistor 40. The cathodes of all the tubes are connected together and the negative side of the anode batteries are connected to the cathodes, as is usual practice. The second grid or screen 18 of each of the 'tubesof the amplifying stages is maintained at itsnormal potential from a suitable volt agetap of the power source 38 at the filament or cathode end of the output or plate circuit. As thus arranged, the resistor 10 is common to the output and input circuits of successive amplifying stages so that the grid of one stage is responsive to variations of potential across the resistorof the immediately preceding such variations of potential.

The normal current in the output circuit causes a normal difference in potential between the terminals of the resistor. This po-' tentialopposes the potential of the power source 38 so far as the effect of'said source on the'grid connected therewith is concerned and, by varying the value of the effective resistance of and thereby the drop across said resistor, the normal or biasing potential of the grid can be adjusted to the normal value up to fifty thousand cycles and with good amplification from fifty thousand up nearly to one hundred thousand cycles without deleterious distortion.

It will be apparent that with this system of amplification, distortion of the amplified signal is due mainly to the presence of stray capacity. WVith the system here shown such capacities are minimized since the usual grid condenser and grid leak is eliminated and the power sources are located in the plate circuits, where the stray capacity between the batteries and the ground, as the side walls of the enclosing casings 20, is not objectionable due to an easily obtained large spaclng between the two. Thus the amplifying system here shown can amplify faithfully over a wide range of frequency and can realize to a higher degree than heretofore a greater part of the high amplification factor provided by the four-element or screened-grid tubes here shown.

The amplifier can be adjusted very easily, itv being necessary only to insert a milli ammeter in the jacks 4A of the plate circuits of the tubes and adjust the resistors 40 until the normal plate current is the same in all tubes.

I claim:

1. A signal amplifying system compris' ing successive amplifying stages each having a thermionic amplifying tube with a grid and an anode and an input and an output circuit, with the output circuit of one stage having a portion which is common to thei'nput circuit ofthe next succeeding stage, the

common portions of said circuits including a source of anode power and a resistor therein which is variable at least in part, the potential drop of which affects said input cirpendent of said input circuit. ing stage and acts to amplify the signals caus- 2. A signal amplifying system including successive stages having thermionic amplifying tubes with input grid circuits and output anode circuits, the output anode circuit of one stage being common in part with the input grid circuit of the next successive stage and having in such common part a source of anode power and in series therewith a device the potential across which is responsive to the normal current in said output circuit and opposes the potential of said power source, whereby the grid of said input circuit said tubes having a common source of heating current, the anode of the tube of one stage having an output circuit between it and'its cathode which includes in series two sources of anode power and-an interposed resistor, the grid of the tube of the next succeeding stage having a direct connection with said output circuit between one of said power sources and said resistor and the circuit of said grid including said reslstor and sald other power source.

4. A signal amplifier having successive amplifying stages each of which has a thermionic amplifying tube provided with a grid, an anode and a cathode, a common source of power for the cathodes of all tubes,

input'circuits associated with the grids, and output circuits associated with the anodes,

the input circuit of one tube being responsive to conditions in the output circuit of the last preceding tube, a source of power in said output circuit, and means located'in said output circuit arranged to maintain the grid of the next succeeding tube at a negative potential which is less than the potential of said power source.

5. A signal amplifier having successive amplifying stages each including a thermionic tube having a grid, a cathode and an anode, input and output circuits associated with said grid and anode respectively, the output circuit of one stage being common in part with the input circuit of the neXt sucs ceeding stage, a source of anode power in the common portionof said input and output circuits,'and means to maintain a biasing potential on the grid associated with said input circuit including a resistor in series with said source of power and both of said circuits, and a second source of potential which is in a part of said out-put circuit between the anode and said resistor that is independent of said grid circuit.

6. A signal amplifier having successive amplifying stages each including a thermionic tube having a grid, a cathode and an anode, input and output circuits associated with said grid and anode respectively, the output circuit of one stage being commonin part with the input circuitof the next succeeding stage, a source of anode power in the common portion of said input and output' circuits, and means to maintain a biasing potential on the grid associated with sa d input circuit including a resistor in series w th said source of. ipowerand in both 1 of said icircui-ts, and a a second source-cf potential which is in a part of 1 said output circuit between the anode andnsaid resistor; that is independent of said grid circuit,isai.d resistor being variable atlea'st-i'n part whereby to vary the'biasing potential of the grid. 5'

7.113s a 'resistance'eoupled amplifier, a c,

three element thermionic tube having a cathode, a grid, and an anode, an output circuit including said cathode and anode, two sources of anode power and an interposed resistor connected in series with said cathode and anode, and a second three element thermionic tube having a grid, a cathode, and an anode, and a grid circuit which includes saidgrid and cathode and said resistor and one of said power sources only of said'output circuit, said other source of anode power being 7 in said output circuit between the anode and said resistor,-and a common source of heating current for the cathodes of both tubes.

8. In a resistance coupled amplifier, a three element thermionic tube having a cathode, a grid, and an anode, an output circuit including said cathode and anode, two sources of anode power and an interposed resistor connected in series with said cathode and anode, and a second three element thermionic tube having a grid, a cathode, and an anode, and a grid circuit which includes said grid and cathode and said resistor and one of said power sources only of said output circuit, the other source of anode power being connected between the anode and said resistor, and the cathodes of both tubes having a common connection with a source of power, the potential of said power source that is in said grid circuit and the-normal potential drop across said resistor providing a suitable grid-biasing potential. I

9. A signal amplifying system including. successive amplifying stages'having thermionic amplifying tubes and grid and anode circuits, the anode circuit of one stage being common in part with thegrid circuit of theneXt succeeding stage, an anode source of power and a resistor in said common part of said circuits, said resistor having such a value of resistance that the potential across it due to the normal anode current traversing it is greater than and opposes the potential of said power source, whereby to subject the grid to a suitable negative biasing potential, and a second source of anode power in a portion of said anode circuit between the ansaid' grid current.

,ode and said resistor that is independent of 10. A high gain amplifier including two cascaded thermionic'tubes the first of which has a shielded grid, the out-put circuit of the shielded grid tubebeing common to part with the input circuit ofthe second tube, a resistor and a source of plate power in such common parts of said circuits, said shielded us I grid having a potential connection with said source of power, and a second source of plate power in a part of said output circuit between the plate and said resistor that is in- 5 dependent of said input circuit.

In testimony whereof, I have signed my name t0 this specification.

HOLLIS S. BAIRD. 

